There are two basic approaches for dealing with carrier synchronisation at a receiver. One approach is to multiplex the modulated signal with a pilot signal and extract the pilot signal at the receiver. The extracted pilot signal is used to synchronise a local oscillator at the receiver. Another approach is to synthesize a carrier signal at the receiver by means of a phase locked loop including a voltage controlled oscillator. The control of the voltage controlled oscillator is effected by a circuit which estimates the carrier phase, the estimation being made from the input modulated signal.
The first approach has the disadvantage that the transmission bandwidth has to include provision for the pilot signal. The second approach has not been applied effectively to a receiver for higher order modulation formats.
The present invention is aimed at providing an improved method and apparatus for the recovery of a carrier phase from a signal modulated in accordance with a higher order M-PSK format.